Methods and apparatus for selectively updating memory cell arrays

ABSTRACT

Methods and apparatus for selectively updating memory cells of a memory cell array are provided. The memory cells of each row of the memory cell array are provided with a plurality of wordlines. Memory cells of the row are activated and updated by separated wordlines. In an application of display systems using memory cell arrays for controlling the pixels of the display system and pulse-width-modulation (PWM) technique for displaying grayscales, the pixels can be modulated by different PWM waveforms. The perceived dynamic-false-contouring artifacts are reduced thereby. In another application, the provision of multiple wordlines enables precise measurements of voltages maintained by memory cells of the memory cell array.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a division of U.S. patent application Ser.No. 10,407,061 to Richards filed Apr. 2, 2003, now U.S. Pat. No.6,856,447, which is a CIP of Ser. No. 10/343,307 filed Jan. 29, 2003(now U.S. Pat. No. 6,962,419), which is US National Phase ofPCT/US01/24332 filed Aug. 3, 2001, which claims priority from Ser. No.09/631,536 filed Aug. 3, 2000 (now U.S. Pat. No. 6,529,310), which is aCIP of Ser. No. 09/437,586 filed Nov. 9, 1999 (now U.S. Pat. No.6,172,797), which is a CON of Ser. No. 09/160,361 filed Sep. 24, 1998(now U.S. Pat. No. 6,046,840) and 60/229,246 filed Aug. 30, 2000, andSer. No. 09/732,445 filed Dec. 7, 2000 (now U.S. Pat. No. 6,523,961),the subject matter of each being incorporated herein by reference in itsentirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is related generally to memory cells, and, moreparticularly, to memory cell arrays used in spatial light modulators.

BACKGROUND OF THE INVENTION

In current memory cell arrays, memory cells in a row of the array areconnected to a single wordline for activating the memory cells. Forexample, in a typical Dynamic Random Access memory (DRAM) cell array asillustrated in FIG. 1, DRAM cells of row 131 are connected to andactivated by wordline 170. A critical constraint on this type of designis that, regardless of the user's intention, the wordline activates allmemory cells of the row simultaneously for writing the intended memorycells during a writing cycle. Consequently, the timing of write eventsis highly correlated. This time-correlation may cause artifacts, such asdynamic-false-contouring (DFC) in display systems that employ memorycell arrays for controlling the pixels of the display systems andpulse-width-modulation (PWM) technique for displaying gray-scales ofimages.

As a way of example, FIGS. 2 a to 2 d illustrate the formation of DFCartifacts in the boundary of two neighboring pixels that are controlledby two neighboring memory cells sharing one wordline. Referring to FIG.2 a, pixels 351 and 353 are two neighboring pixels of the display systemand are controlled by two neighboring memory cell, such as memory cells191 and 193 in FIG. 1. Assuming that gray-scaled images of an objecttraversing from left to right are to be displayed by the two pixels,illumination intensities of the two pixels are modulated using PWMwaveforms such that, in the screen (pixel) coordinate, the averagedillumination intensity over a frame duration T of each pixel correspondsto the desired grayscale of the image. As viewed by stationary humaneyes, the difference of the averaged illumination intensity at theboundary of the two pixels is perceived as the contour of the object, asshown in FIG. 2 b.

However, the contour of the object will be distorted in the retinacoordinate in viewer's eye when the eyes move with the object. FIG. 2 cpresents the two pixels in the retina coordinate that moves with theeyes and the object. As can be seen, the pixels are distorted. Theboundary of the two pixels is extended into a region, in which theaveraged illumination intensity varies with position, as shown in FIG. 2d. This variation of the averaged illumination intensity will beperceived and recognized by the eyes as “real” contour of the object.This phenomenon is generally referred to as DFC artifact.

Therefore, methods and apparatus are desired for decorrelating thememory cells and associated pixels of a spatial light modulator suchthat the DFC like artifacts can be effectively reduced, if notremovable.

SUMMARY OF THE INVENTION

In view of the forgoing, the present invention provides a method and anapparatus for selectively updating memory cells in each row of thememory cell arrays such that the update events of neighboring memorycells are decorrelated in time. As a result, the pixels corresponding tothe memory cells are also time-decorrelated.

In an embodiment of the invention, a method is disclosed herein. Themethod comprises: providing a memory-cell array comprising a pluralityof memory cells; and activating the memory cells of a row of the arrayusing a plurality of separate word lines of the row such that at leasttwo memory cells of the row are activated by separate word lines.

In another embodiment of the invention, a method for displaying agray-scale image is disclosed herein. The method comprises: providing aspatial light modulator comprising an array of pixel elements; definingat least a first and a second waveform format based on apulse-width-modulation technique; defining at least a first set ofwaveforms according to the first waveform format and the gray scale ofthe image; defining at least a second set of waveforms according to atleast the second waveform format; updating the pixels of a row of thearray in accordance with a plurality of waveforms that are selected fromthe first and second sets of waveforms such that at least a first pixelof the row is written in accordance with at least a first waveformselected from the first set of waveforms, and at least a second pixelother than the first pixel of the row is written in accordance with atleast a second waveform selected from the second set of waveforms.

In yet another embodiment of the invention, a system is provided herein.The system comprises: a memory-cell array comprising a plurality ofmemory cells; and a plurality of word-lines coupled to the memory cellsof a row of the memory-cell array for selectively activating the memorycells such that at least two memory cells of the row are coupled toseparate word-lines of the plurality of word-lines.

In a further embodiment of the invention, a display system fordisplaying a gray-scale image on a target is proved herein. The displaysystem comprises: a light source; a spatial light modulator that employsa pulse-width-modulation technique for displaying the image byreflecting a beam of incident light from the light source andselectively directing the reflected light to the target, the spatiallight modulator further comprising: a plurality of micromirrors forselectively reflecting the beam of incident light onto the target; amemory-cell array having a plurality of memory cells for storing a setof information for controlling the deflections of the micromirrors; anda plurality of word lines coupled with the memory cells of a row of thememory-cell array for activating the memory cells for updating thestored information such that at least two different memory cells of therow can be actuated by separate word lines of the plurality of wordlines.

In still a further embodiment of the invention, a method for displayinga gray-scale image on a target is disclosed herein. The methodcomprises: defining a set of separate waveforms in accordance with atleast a gray-scale information of the image and based on apulse-width-modulation technique; directing an incident light onto amicromirror array that has a plurality of deflectable reflectivemicromirrors; and selectively reflecting, by the micromirror array, theincident light onto the target according to the set of separatewaveforms such that at least two different micromirrors of a row of thearray reflect the incident light according to at least two separatewaveforms.

In yet another embodiment of the invention, a method for displaying animage is disclosed herein. The method comprises: providing a spatiallight modulator having rows and columns of pixels in an array;addressing pixels within a row of the array by providing a brightnesslevel to each pixel in the row, the brightness level being achieved byactivating each pixel with a series of bits of varying differentlengths, wherein the combination of “on” bits during a frame correspondsto a brightness level for each pixel; and wherein the order of theseries of bits for each pixel in a row is not the same or the weightingsof the series of bits are different.

In still yet another embodiment of the invention, a method fordisplaying an image is disclosed herein. The method comprises: providinga spatial light modulator having rows and columns of pixels in an array;addressing pixels within a row of the array by providing a brightnesslevel to each pixel in the row, the brightness level being achieved byactivating each pixel with a series of bits of different lengths,wherein a plurality of pixels in the row have the same brightness levelbut a different combination of “on” and “off” bits during a frame.

In yet another embodiment of the invention, a spatial light modulator isprovided herein. The spatial light modulator comprises: a plurality ofrows and columns of pixels in an array; a bit line for each column; anda plurality of word lines for a row of the plurality of rows.

BRIEF DESCRIPTION OF THE DRAWINGS

While the appended claims set forth the features of the presentinvention with particularity, the invention, together with its objectsand advantages, may be best understood from the following detaileddescription taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 presents a typical memory-cell array in prior art;

FIG. 2 a through 2 d illustrate a perceived dynamic-false-contouringartifact at the boundary of two neighboring memory cells; wherein FIG. 2a presents the two neighboring cells showing grayscales of a movingobject in the screen coordinate; wherein FIG. 2 b the shows perceivedillumination intensity by the eyes of the shown grayscales of FIG. 2 ain the screen coordinate; wherein FIG. 2 c presents the two neighboringcells showing grayscales in the retina coordinate that moves with themoving object; and wherein FIG. 2 d presents the perceived illuminationintensity of the shown grayscales of FIG. 2 c in the retina coordinate;

FIG. 3 shows a simplified display system that employs a MEMS-basedspatial light modulator;

FIG. 4 a illustrates a micromirror array having duel wordlines for eachrow of the memory cells according to an embodiment of the invention, andFIG. 4 b illustrates a sub-array of the memory cell array of FIG. 4 a;

FIG. 5 a illustrates a row of pixels displaying gray-scaled images of amoving object in the screen coordinate;

FIG. 5 b illustrates a row of prior art pixels viewed by viewer eyes,the pixels showing a gray-scaled image of a moving object, and theviewer eyes following the motion of the moving object;

FIG. 5 c illustrate the perceived illumination intensity of the pixelsin FIG. 5 b;

FIG. 6 a demonstrates a 4-bits binary-weighted waveform format;

FIG. 6 b and FIG. 6 c illustrate two exemplary binary-weightedpulse-width-modulation waveforms generated according to the waveformformat in FIG. 6 a;

FIG. 7 shows another exemplary binary-weighted waveform format accordingto another embodiment of the invention;

FIG. 8 a and FIG. 8 b present two exemplary waveforms generatedaccording to the waveform format in FIG. 7;

FIG. 9 a present yet another exemplary waveform format according to yetanother embodiment of the invention;

FIG. 9 b presents a further exemplary waveform format according to afurther embodiment of the invention;

FIG. 10 a illustrates a row of pixels viewed by viewer eyes, the pixelsshowing a gray-scaled image of a moving object according to anembodiment if the invention, and the viewer eyes following the motion ofthe moving object

FIG. 10 b demonstrate the perceived illumination intensity of the pixelsin FIG. 10 a;

FIG. 11 illustrates a memory cell array having dual wordlines for eachrow of the memory cell array according to another embodiment of theinvention;

FIG. 12 illustrates a memory cell array having dual wordlines for eachrow of the memory cell array according to yet another embodiment of theinvention; and

FIG. 13 is a diagram demonstrating a method for measuring the state of amemory cell in a memory cell array according to a further embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method and an apparatus for selectivelyupdating memory cells in a row of a memory cell array by providingmultiple wordlines to each row such that the timing of update events tomemory cells in the row are decorrelated. In display systems employingmemory cells for controlling the pixels of the display system andpulse-width-modulation technique for generating grayscales or color, thedecorrelation of the memory cells reduces correlation of neighboringpixels. The dynamic-false-contouring artifacts are thereby reduced. Inanother application, the multiple wordlines of each row of the memorycell array enable read back of voltages stored in memory cells,enhancing the device's testability.

To selectively update memory cells of a row of a memory cell array, thememory cells of the row are divided into subgroups according to apredefined criterion. For example, neighboring memory cells in a row aregrouped into separate subgroups. For another example, the positions ofthe memory cells in a row in different subgroups are interleaved. Aplurality of wordlines is provided for each row of the memory cellarray. The memory cells are connected to the plurality of wordlines suchthat memory cells in the same subgroup are connected to the samewordline, and memory cells in different subgroups are connected toseparate wordlines. With this configuration, memory cells in differentsubgroups are activated or updated independently by separate wordlines.Memory cells in different subgroups of the row can be activatedasynchronously or synchronously as desired by scheduling the activationevents of the wordlines. Moreover, memory cells in different rows of thememory cell array can be selectively updated asynchronously orsynchronously as desired. For example, one can simultaneously updatememory cells in a subgroup (e.g. even numbered memory cells) of a rowand memory cells in another subgroup (e.g. odd numbered memory cells) ofa different row. Of course, memory cells in different subgroups ofdifferent rows can be activated at different times.

In an application of display systems that employ memory cell arrays forcontrolling the pixels of the system and pulse-width-modulationtechnique, perceived artifacts, such as dynamic-false-contouring (DFC)artifacts can be reduced, if not removed. To attain this purpose, theoriginal DFC artifacts, which are formed at the boundaries ofneighboring pixels having different gray scales, are intentionallyreproduced and distributed over the entire pixel row. As a result, thereproduced and the original DFC artifacts are redistributed at a higherspatial frequency. That is, the original DFC artifacts will no longer berecognized by viewer as the “real” contour of the object.

To redistribute the DFC artifacts, illumination-intensity modulationsare performed differently at the neighboring pixels of the row. As aresult, the averaged illumination intensity at each boundary ofneighboring pixels varies and forms DFC artifacts. In order to modulatethe neighboring pixels in different ways, the memory cells controllingthe neighboring pixels are expected to be activated or updatedindependently. The present invention provides multiple wordlines for thememory cells in each row of the memory cell array. This enables thememory cells controlling the selected neighboring pixels to be connectedto and activated by separate wordlines.

Embodiments of the present invention can be implemented in a variety ofways and systems, such as optical switches and display systems. In thefollowing, embodiments of the present invention will be discussed in adisplay system that employs micromirror arrays and PWM technique,wherein individual micromirrors of the micromirror array are controlledby memory cells. For clarity and demonstration purposes without losinggenerality, the embodiments will be illustrated using a simplified 4-bitgrayscale on a memory cell array. It will be understood that theembodiments of the present invention are applicable to any grayscale orcolor pulse-width-modulation waveform, such as those described in U.S.Pat. No. 6,388,661, and U.S. patent application Ser. No. 10/340,162,filed on Jan. 10, 2003, both to Richards, the subject matter of eachbeing incorporated herein by reference. Each memory cell of the arraymay be a 1T1C (one transistor and one capacitor) circuit. Each row ofthe memory cell array is provided with two wordlines. It will beapparent to one of ordinary skill in the art that the followingdiscussion applies generally to other types of memory cells, such asDRAM, SRAM or latch. The wordlines for each row of the memory array canbe of any suitable number equal to or larger than two. Other PWMwaveforms (e.g. other bit-depths and/or non binary weightings) may alsobe applied. Furthermore, although not limited thereto, the presentinvention is particularly useful for operating micromirrors such asthose described in U.S. Pat. No. 5,835,256, the contents of which arehereby incorporated by reference.

Referring to FIG. 3, a simplified display system, in which embodimentsof the presented invention may be implemented, is illustrated therein.The display system employs a spatial light modulator (hereafter, SLM)and pulse-width-modulation technique. A light source 210 and associatedoptical devices, such as light pipe 250, optical lens 270, focus a lightbeam onto SLM 350. The pixels of SLM are individually controllable andan image is formed by modulating the incident light beam as desired ateach pixel. Modulated light from each SLM pixel passes throughprojection lens 330 and is projected onto display target 310, whichshows an image composed of bright and dark pixels corresponding to theimage data loaded into the SLM. For displaying color images, color wheel230 is provided as shown in the figure.

In order to produce the perception of a gray-scale or full color imagein such a display system, it is necessary to rapidly modulate the pixelsbetween “ON” and “OFF” states such that the average over a time period(e.g. the time period corresponds to the critical flicker frequency) oftheir modulated brightness corresponds to the desired “analog”brightness for each pixel. This technique is generally referred to aspulse-width-modulation (PWM). Above a certain modulation frequency, theviewer eyes and brain integrate a pixel's rapid varying brightness andperceived brightness determined by the pixel's average illumination overa period of time. The modulation of illumination of pixels is controlledby a memory cell array associated with the pixels of the display system.

Referring to FIG. 4 a, a memory cell array according to an embodiment ofthe invention is illustrated therein. The memory cell array has twowordlines for each row of the array, and memory cells of a row areconnected to separate wordlines. For example, memory cell row 500 hastwo separate wordlines 510 and 530. Neighboring memory cells of each roware connected to separate wordlines. Specifically, odd numbered memorycells are in one subgroup, and even numbered memory cells are in anothersubgroup. Memory cells in different subgroups are connected to separatewordlines. For example, memory cells 501 and 502 are respectivelyconnected to wordlines 530 and 510. Memory cells in the same subgroupare connected to the same wordline. For example, memory cells 501 and503 (or memory cells 502 and 504) are connected to wordline 530 (or510). With this configuration, neighboring memory cells can be activatedseparately. The time-correlation between neighboring pixels in currentmemory cell arrays can thus be removed. In an aspect of the invention,neighboring memory cells can also be activated asynchronously orsynchronously as desired by properly scheduling the activation events ofthe wordlines. For example, memory cell 501 can be activated earlier viawordline 530 than memory cell 502 via wordline 510. Of course, the twowordlines can be synchronized, and all memory cells in the row (e.g. row500) can be activated at the same time by synchronizing the wordlines.

FIG. 4 b is a sub-array of the memory cell array in FIG. 4 a. As can beseen in FIG. 4 b, word lines (e.g. lines 420, 422, 430 and 432 extendinghorizontally in the figure) form a “grid” with the bit lines (e.g. lines441, 442, and 443 extending vertically in the figure). However, as canalso be seen in this figure, each word line does not connect at a memorycell with each bit line such that the number of bit lines times thenumber of word lines for addressing the micromirrors is greater than thetotal number of micromirrors. As an example, in FIG. 4 b, 4 word linestimes 3 bit lines=12, however the number of micromirrors addressed inthe figure is 6. More particularly, as seen in FIG. 4 b, each word lineconnects at a memory cell with every other bit line, and each bit lineconnects at a memory cell with every other word line. For example, wordline 432 connects with bit line 441 at memory cell 411 for activatingmemory cell 411. Word line 432 however “skips” bit line 442 and memorycell 410, but does connect with the next bit line 443 at memory cell 412to activate memory cell 412.

As further seen in this FIG. 4 b, for two adjacent word lines extendinghorizontally (e.g. word lines 430 and 432), one word line (e.g. wordline 430) connects with memory cells of a first group (e.g. memory cell410), whereas the adjacent word line (e.g. word line 412) connects withmemory cells of a second group (e.g. memory cells 411 and 412), wheresuch first and second groups are not aligned vertically but instead areinterleaved in a horizontal direction. In comparison, in the prior artsuch as illustrated in FIG. 1, any two word lines that are adjacent eachother (extending in a horizontal direction) connect to memory cells thatare aligned in the vertical direction.

Under the control of the memory cell array in FIG. 4 a, DFC artifactscan be reduced in the display system in FIG. 3. For simplicity andillustration purposes, gray-scaled images of an object that moves fromleft to right are to be displayed by a row of pixels of the spatiallight modulator 350 in FIG. 3. Referring to FIG. 5 a, the pixel row has17 pixels numbered from 1 to 17. Each of the pixels is associated with amemory cell of row 500 in FIG. 4 a for electrostatically controlling thepixel.

In order to simulate grayscales of the moving object, PWM waveforms aregenerated according to the predefined PWM waveform formats and thedesired grayscales. In the embodiment of the invention, at least twobinary-weighted PWM waveform formats are defined. A first PWM waveformformat is a binary-weighted waveform format starting from the leastsignificant bit (LSB) and ending at the most significant bit (MSB), asshown in FIG. 6 a. A second PWM waveform format is a binary-weightedwaveform format starting from the MSB and ending at the LSB, as shown inFIG. 7. Though preferred, other suitable waveform formats could also beapplied. In particular, the waveform format can be a binary-weightedformat with the binary weights randomly arranged, as shown in FIG. 9 a.Alternatively, the waveform format can be non-binary weighted format, asshown in FIG. 9 b.

Given the defined waveform formats, PWM waveforms are generatedaccording to the desired grayscales. For example, PWM waveforms shown inFIGS. 6 b and 6 c are generated based on the defined format of FIG. 6 a.And PWM waveforms shown in FIGS. 8 a and 8 b are generated based on thedefined format of FIG. 7. Referring to FIG. 6 b, the waveform is in the“OFF” state during the first 7 (7=1+2+4) segments of the frame durationT and turned “ON” for the rest 8 segments. Referring to FIG. 6 c, thewaveform presented therein is turned “ON” for the first 3 (3=1+2)segments of the frame duration T and turned “OFF” for the rest 12(12=4+8) segments. By feeding the waveforms shown in FIGS. 6 b and 6 cinto the pixels in FIG. 5 a, illumination intensities of the pixels aremodulated over the frame duration T. Specifically, within the firstduration T, pixels 1 through 9, and 14 through 17 are turned “OFF”(dark) during the first 7 segments of the frame duration T. These pixelsare then turned “ON” (bright) for the rest 8 segments. Pixels 10 through13 are first turned “ON” for the first 3 waveform segments and turned“OFF” for the rest 12 segments. The modulation is repeated for thefollowing frame duration (e.g. from T to 2T). In this way, illuminationintensities are distributed over the 17 pixels during the frame durationT. This illumination pattern, however, is distorted in the retinacoordinate of viewer's eyes that moves with the moving object, as shownin FIG. 5 b.

Referring to FIG. 5 b, DFC artifacts are generated at the boundaries ofpixels having different illumination intensities. Specifically, pixels 9and 10 have different distribution of illumination intensities. Theaveraged illumination intensity, thus the perceived illuminationintensity, varies in the boundary of the two pixels, as show in FIG. 5c. This variation is perceived by the viewer's eyes as the “real”contour of the object. For the same reason, another DFC artifact isgenerated at the boundary of pixels 13 and 14.

In order to reduce these perceived DFC artifacts, these original DFCartifacts are intentionally reproduced between selected pixels anddistributed over the pixel row. To attain this purpose, a second set ofPWM waveforms, which is different from the first set of waveformscorresponding for driving the pixels to display desired grayscales, isgenerated. In the embodiment of the invention, a second set of PWM isgenerated based on a second PWM waveform format, as shown in FIG. 7. Thesecond waveform format is a binary-weighted waveform format startingfrom the MSB and ending at the LSB. FIGS. 8 a and 8 b show two exemplaryPWM waveforms generated based on such waveform format. Referring to FIG.8 a, the waveform is in “ON” state for the first 8 segments of the frameduration T and turned “OFF” for the rest 7 segments. Referring to FIG. 8b, the waveform is “OFF” for the first 12 segments of the frame durationT and turned “ON” for the rest 3 segments. The generated waveforms inFIGS. 6 b, 6 c, 8 a and 8 b are applied concurrently for driving thepixels of the row for reproducing the DFC artifacts, as shown in FIG. 10a.

Referring to FIG. 10 a, odd numbered pixels 1 to 9, 15 and 17 are drivenby the waveform in FIG. 6 b. Odd numbered pixels 11 and 13 are driven bythe waveform in FIG. 6 c. Because these waveforms are generatedaccording to the desired grayscales of the images, the perceivedgrayscales of these odd numbered pixels by viewer's eyes correspond tothe desired grayscales of the images. To reproduce the DFC contouring,the waveform in FIG. 8 a is applied to the even numbered pixels 2 to 8,14 and 16. And the waveform in FIG. 8 b is applied to the even numberedpixels 10 and 12. As a consequence, neighboring memory cells aremodulated with different waveforms. The averaged illumination intensityvaries in each boundary of neighboring odd and even numbered pixels, asshown in FIG. 10 b.

Referring to 10 b, DFC artifacts are reproduced in the pixel row. As canbeen seen, the illumination intensity varies in a small range relativeto background illumination intensities, represented by dash lines in thefigure. The background illumination intensities correspond to theaveraged illumination intensities, shown in FIG. 5 c, and desiredgrayscales of the image. Because of this, the reproduced DFC artifactsare perceived as background “noise” by the viewer.

As described above, the pixels are selectively modulated with differentwaveforms. This modulation is controlled by the memory cells of row 500in FIG. 4. Because the neighboring memory cells are connected to andcapable of being activated by separate wordlines, the associatedneighboring pixels can be driven independently by separate waveforms.For example, odd numbered memory cells are activated by wordline 530.The odd memory cells 1 to 9, 15 and 17 can be written according to thePWM waveform in FIG. 6 b, and the odd numbered memory cells 11 and 13can be written according to the PWM waveform in FIG. 6 c. Specifically,the odd numbered memory cells 1 to 9, 15 and 17 are set to a voltagestate corresponding to the “OFF” state of the pixels for the first 7segments of the frame duration T, and set to another voltage statecorresponding to the “ON” state for the rest 8 segments. The memorycells 11 and 13 are set to the same voltage state corresponding to the“OFF” state for the first 3 segments of the frame duration and set tothe same another voltage state corresponding to the “ON” state of thepixels. In this way, the odd numbered pixels associated with the oddnumbered memory cells are tuned “ON” and “OFF” according to the desiredwaveforms for displaying the desired grayscales of the images.

Independent from the activation and update of the odd numbered memorycells, the even numbered memory cells are activated and updated bywordline 510 in FIG. 4. The even numbered memory cells 2 to 8, 14 and 16are then written according to the PWM waveform in FIG. 8 a, and the evennumbered memory cells 10 and 12 are written according to the PWMwaveform in FIG. 8 b. Specifically, the even numbered memory cells 2 to8, 14 and 16 are set to the voltage state corresponding to the “ON”state for the first 8 waveform segments of the frame duration T and setto the voltage state corresponding to the “OFF” state for the rest 7segments. The even numbered memory cells 10 and 12 are set to thevoltage state corresponding to the “OFF” state for the first 12 segmentsof the frame duration T and to the voltage state corresponding to the“ON” state for the rest 3 segments. It can be seen that the even and oddnumbered memory cells are activated and written independently, theassociated even and odd numbered pixels are thus updated independentlyand driven by separate waveforms. Thereby, differences of illuminationintensities, and thus DFC artifacts, are created at the boundaries ofeven and odd numbered pixels.

In the above described embodiments, the memory cells of each row of thememory cell array are grouped such that neighboring memory cells are indifferent subgroups and connected to separate wordlines. According toanother embodiment of the invention, the memory cells of each of thememory cell array are grouped such that the positions of the memorycells in different subgroups are interleaved in the row, as shown inFIG. 11. Referring to FIG. 11, memory cells 601, 602, 605 and 606 of row600 are in the same subgroup and are connected to the same wordline(e.g. wordline 610). Memory cells 603, 604, 607 and 608 of row 600 aregrouped in another subgroup and are connected to wordline 612 that isseparate from wordline 610.

In yet another embodiment of the invention, memory cells of each row ofthe memory cell array are grouped randomly and at least two memory cellsin the same row are grouped into different subgroups, as shown in FIG.12.

As another application, the provision of multiple wordlines for each rowof a memory cell array enables readout of voltages maintained by memorycells of spatial light modulators, as shown in FIG. 13. Referring toFIG. 13, the voltage V_(m) maintained by memory cell 551 is to bemeasured. In general, V_(m) is difficult to be measured precisely anddirectly (e.g. directly measure the voltage drop across the capacitorC₀). This arises from facts that C₀ is much smaller than the distributedcapacitor C_(d), and V_(m) (maintained by C₀) is superpositioned withV_(d) (maintained by C_(d)) that is much larger than V_(m). C_(d) is adistributed capacitor that is formed for example, by parasiticcapacitance of the bitlines. An efficient way to precisely measure V_(m)is to extract V_(m) from the large voltage background V_(d) using adifferential amplifier (e.g. 580 in FIG. 13). The differential amplifierconcurrently measures a signal from the memory cells to be measured anda reference voltage signal. The large voltage background (e.g. V_(d))that is common to the two signals will be removed from the two measuredsignals, and the small difference of the two voltage signals isextracted and amplified for precise measurement. Given the referencesignal, the small voltage V_(m) can thus be determined.

According to an embodiment of the invention, memory cell 551 that is tobe measured and memory cell 571 that is adjacent to memory cell 551 aretwo memory cells of a spatial light modulator (not shown) and arerespectively connected to wordlines 550 and 570. In a measurement,memory cell 571 is activated by wordline 570 and set to a referencevoltage V_(r) corresponding to a predefined reference state. As anexample, assuming that the “ON” state of the memory cell corresponds to+15V and the “OFF” state corresponds to 0V, then reference voltage V_(r)can be +7.5V (7.5=15/2), represented by “½” state. Then, memory cell 551is independently activated by wordline 550. Differential amplifier 580measures voltage signals from memory cells 551 and 571, and extracts thedifference of V_(m) and V_(r). Because V_(r) is set to a known referencevoltage (+7.5V), V_(m) can thus be determined. Based on the comparisonof V_(m) and V_(r), the state of memory cell 551 can also be determined.For example, if the absolute value of V_(m) is larger than the absolutevalue of V_(r), memory cell 551 is said to be in the “ON” state.Otherwise, memory cell 551 is said to be in the “OFF” state.

It will be appreciated by those of skill in the art that a new anduseful method and apparatus for selectively updating memory cells of amemory cell array have been described herein. In view of the manypossible embodiments to which the principles of this invention may beapplied, however, it should be recognized that the embodiments describedherein with respect to the drawing figures are meant to be illustrativeonly and should not be taken as limiting the scope of invention. Forexample, those of skill in the art will recognize that the illustratedembodiments can be modified in arrangement and detail without departingfrom the spirit of the invention. Although the invention is describedwith reference to DRAM memory cells in display systems employing SLM,those skilled in the art will recognize that such may be equivalentlyreplaced by any suitable memory cells, such as charge-pump pixel cell(described patent application, Ser. No. 10,340,162, filed on Jan. 10,2003 to Richards), SRAM or latch and optical switches using SLM. Though4-bits binary-weighted PWM waveform formats are used in describing theembodiments of the invention, this should not be interpreted aslimitations of the invention. For example, 128 bits or 256 bitsweightings could be applied. Instead, any suitable PWM waveforms areapplicable for driving the pixels of the display system. Therefore, theinvention as described herein contemplates all such embodiments as maycome within the scope of the following claims and equivalents thereof.

1. A method for operating a memory cell array of a spatial lightmodulator of a display system, the memory cell array comprising aplurality of memory cells, the method comprising: grouping the memorycells in each row of the memory cell array into a set of subgroupsaccording to a predefined criterion, each subgroup comprising at leastone memory cell; connecting the memory cells to a plurality of wordlinessuch that memory cells in the same subgroup are connected to the samewordline, and memory cells in different subgroups are connected toseparate wordlines; activating the memory cells in a first subgroupconnected to a first wordline of a first row of the memory cell array;and activating the memory cells in a second subgroup connected to asecond wordline of a second row of the memory cell array; wherein thememory cells of a first subgroup of the subgroups are connected to afirst subset of the plurality of bitlines, and the memory cells of asecond subgroup are connected to a second subset other than the firstsubset of the plurality of bitlines.
 2. The method of claim 1, whereinthe activation of the memory cells in the first subgroup and theactivation of the memory cells in the second subgroup are performedsimultaneously.
 3. The method of claim 1, wherein the first subgroupcomprises the even numbered memory cells of the first row; and whereinthe second subgroup comprises the odd numbered memory cells of thesecond row.
 4. A projector, comprising: a light source; a spatial lightmodulator comprising: an array of micromirrors; and a plurality ofwordlines, wherein four wordlines connect to a 2×3 sub-array of themicromirror array; and wherein each micromirror is connected to andactivated by one wordline such that the micromirrors are capable ofbeing updated by separate wordlines; a set of optical elements; and adisplay target.
 5. The projector of claim 4, wherein each wordline ofthe plurality of wordlines connects the micromirrors in the same row. 6.The projector of claim 4, wherein each wordline of the plurality ofwordlines connects every the other micromirrors in a row of themicromirror array.
 7. A display system comprising: a light source; aspatial light modulator, comprising: an array of pixels each comprisinga deflectable reflective micromirror and associated with a memory celland electrode for electrostatically deflecting the micromirror; and aplurality of word lines and a plurality of bit lines for activatingmemory cells so as to deflect an associated micromirror; wherein eachword line of the plurality of word lines does not connect at a memorycell with every bit line: and wherein a dynamic false contouring (DFC)artifact is capable of being reduced as compared to a spatial lightmodulator where each word line connects at a memory cell with every bitline.
 8. The display system of claim 7, wherein each word line connectsat a memory cell with every other bit line.
 9. The display system ofclaim 7, wherein each bit line connects at a memory cell with everyother word line.
 10. The display system of claim 7, wherein each memorycell comprises a transistor and a capacitor.
 11. The display system ofclaim 7, wherein each memory cell is an SRAM.
 12. The display system ofclaim 7, wherein each memory cell is a DRAM.
 13. The display system ofclaim 7, wherein the micromirrors are capable of being operated by pulsewidth modulation to obtain a gray scale image.
 14. The display system ofclaim 7, wherein the number of bit lines times the number of word linesis greater than the number of micromirrors.
 15. The display system ofclaim 7, wherein in a 2×3 array of the micromirrors, 4 word linesconnect to memory cells of the 2×3 array.
 16. A spatial light modulator,comprising: an array of pixels each comprising a deflectable reflectivemicromirror and associated with a memory cell and electrode forelectrostatically deflecting the micromirror; a plurality of wordlinesand a plurality of bit lines for activating memory cells so as todeflect an associated micromirror in such a way that the micromirrorsare capable of being deflected by separate wordlines; and wherein eachword line of the plurality of word lines does not connect at a memorycell with every bit line.
 17. A method for operating a spatial lightmodulator, comprising: providing a spatial light modulator thatcomprises: an array of pixels each comprising a deflectable reflectivemicromirror and associated with a memory cell and electrode forelectrostatically deflecting the micromirror; and a plurality of wordlines and a plurality of bit lines for activating memory cells so as todeflect an associated micromirror; wherein each word line of theplurality of word lines does not connect at a memory cell with each bitline; and providing a first set of PWM waveforms to micromirrorsassociated with a first group of the plurality of word lines, andproviding a second set of PWM waveforms to micromirrors associated witha second group of the plurality of word lines, where the second set ofPWM waveforms is different from the first set of waveforms.
 18. Adisplay system comprising: a light source providing illumination lightfor the display system; a spatial light modulator, comprising: an arrayof micromirrors for modulating the illumination light; and a set ofhorizontal lines in connection with the micromirrors for addressing themicromirrors in a way such that different micromirrors are capable ofbeing addressed by different horizontal lines, wherein each of the setof horizontal lines connects every other micromirror of the array; and adisplay target on which the modulated illumination light is projected.19. The display system of claim 18, wherein the horizontal lines arewordlines.
 20. The display system of claim 19, further comprising: a setof vertical lines in connection with the micromirrors of the array. 21.The display system of claim 20, wherein the vertical lines are bitlines.